"Then, Sir, the stars, being so very far off, appear so very little, and the rays of light they send are disturbed by atmospheric vapor, and thus to the naked eye they twinkle."
"Yes," said the Professor. "The sun and moon, as you know, present broad disks, with light radiating from every point--"
"Please, Professor," interrupted May, "tell us what a 'disk' is. Jack's big words are dreadful to understand; and this, although a little one, seems just as bad."
"I don't wonder I puzzled you, May. We use the word disk for the face or surface of a heavenly body which appears to have some size. You may always stop me when I use a word you don't understand; but when I have once told you, I shall want you to remember; for we can not know much about science unless we learn some of the hard words. I was saying that the sun and moon present broad disks, so that if some of the light is intercepted, the eye does not notice the loss. The same is true also of the planets, which appear large when they are magnified, but not of the stars, owing to their immense distances; and when the impurities in the atmosphere obscure or divert the narrow line of light they send to us, the eye perceives it at once. Some of the stars appear very brilliant through the large telescopes, but the light still seems to proceed from a single point. There are some four or five thousand stars that can be seen without a telescope."
"Why," interrupted Joe, "I thought there were more than anybody could count."
"So there are," replied the Professor, "but the number that can be distinctly perceived by the unassisted eye is found to be comparatively small when they are carefully looked after. On very clear nights the whole sky may seem to glisten when the eye is suddenly turned upward; and there are some portions of it where a confused light comes from a sort of star-cloud, which has received the name of 'Milky Way.' But the stars that can be seen separately are very easily counted. Some persons can see rather more than others, on account of their eye-sight being naturally better, or improved by use. A rough count of the number that could be seen through Herschel's famous telescope made it twenty thousand. The great telescopes more recently made would probably show as many as forty or fifty millions."